Igniters are necessary in many applications including the welding of parts using exothermic mixtures. A common way of making welds or joints is to provide a split graphite mold including a crucible and enclose in the mold the parts to be joined. The parts to be joined are first cleaned and placed appropriately in the graphite mold. The mold is then closed and locked and a metal disc is positioned in the crucible over the tap hole leading to the weld cavity in the mold. The necessary amount of exothermic material is then placed into the crucible section of the mold and a starting material, essentially a much finer exothermic mixture, is sprinkled over the top. The mold cover is then closed, and the starting material is ignited using a flint igniter. When the material is ignited, the molten metal formed separates from the slag, drops to the bottom of the crucible and melts the metal disc. The molten metal then drops through the tap hole, melts and joins the parts together. The molten metal is allowed to cool and the mold is opened. The slag material which stays above the weld in the crucible is separated from the weld or joint and the mold is cleaned for use on another connection. Some examples of welding apparatus are the CADWELD.RTM. process and the THERMIT process. CADWELD is a trademark of Erico Products, Inc., Solon, Ohio, U.S.A. and THERMIT is a trademark of Th. Goldschmidt AG, Essen, W. Germany.
Exothermic mixtures consist of powdered metals, usually a transition metal oxide and a reductant metal, e.g. copper oxide and aluminum which upon ignition supplies enough heat to sustain and propagate a reaction within the mixture. The heat and/or products of this reaction is then used to produce a desired result. For example, the reaction between aluminum and copper oxide, in the CADWELD process, produces molten copper and aluminum oxide. The molten copper is of a higher density and is usually directed, by a mold, to join or weld copper or steel together. The aluminum oxide becomes slag and is broken off the weld or the joint and discarded.
Exothermic mixtures do not ignite spontaneously and a method of igniting this mixture is necessary to generate enough localized energy to permit the reaction to begin. Once the reaction has begun it becomes self-sustaining and requires no further energy to proceed to completion. Many combinations of transition metal oxides and reductant metals may react exothermically, depending on the conditions. The two most common mixtures are copper oxide with aluminum, and iron oxide with aluminum.
Some situations may require the remote ignition of the mixture. Also, the handling, shipping and preparatory process for the use of the starting material makes desirable an exothermic ignition process which would avoid the use of starting material. The development of filter systems such as shown in Brosnan et al application Ser. No. 128,597, filed Dec. 4, 1987 now U.S. Pat. No. 4,889,324, entitled "Exothermic Welding Apparatus And Method" may require remote ignition. Also, the development of self-contained welding packages such as those disclosed in the copending application of Kovaric et al entitled "Exothermic Welding Apparatus And Method", filed Apr. 4, 1988, Ser. No. 177,075 now U.S. Pat. No. 4,879,452, may cause the conventional ignition methods to be unworkable.
Exothermic materials may be ignited by a hot wire process, however such processes have several drawbacks. One of these drawbacks is reliability. In order for ignition to occur, the wire must transfer enough heat to the material before it fuses. If the wire fuses and ignition doesn't occur, the wire has to be replaced, reconnected and properly positioned in the exothermic material. Coils, lengths, or other patterns may be employed throughout the mixture such as seen in French patent specification 324,534 to Weber. However, because of the fusion of the wire, ignition, if it occurs at all, will normally occur only at a single point in the mixture. Single point ignition may be acceptable in some circumstances, however, to produce a quality weld multiple ignition points are necessary.
Other attempts at electrical ignition have been made, one utilizing highly esoteric and, of course, expensive solid rocket igniters such as shown in the copending application of Brosnan et al, Ser. No. 030,169 now abandoned. Such igniters operate at relatively low temperatures and are quite costly.
In a proper process, a delay occurs between ignition and the dropping of the molten metal into the weld chamber. Such delay results in completion of the reaction for all of the mixture and also permits any slag formed to rise to the top of the molten metal. Thus any slag formed will be drawn into the weld cavity last and will collect in a riser which may subsequently be removed. Thus if the disk melts through prematurely, slag or unreacted exothermic material or both may be drawn into the critical portion of the weld cavity, adversely affecting the quality of the weld.
It is therefore desirable to provide a reliable low cost remote electrical ignition system for exothermic materials which does not require starting material. Additionally it is desirable to provide an ignition which utilizes a lightweight and hand portable power source. Furthermore, an igniter preferably should not ignite at too low a temperature because of problems in shipping and handling. Still further, it is important that when the process is used to form welds, it is compatible with a system in which it is necessary for ignition to occur over the top surface of the mixture so that the problems noted above with single point ignition are avoided.